The present invention relates to processes for inducing pre-tilt in alignment of a liquid crystal medium, compositions useful in these processes, and liquid crystal display elements.
Liquid crystal compounds are used in human and machine readable displays, finding applications in instrument controls, such as those in motor vehicles, avionics, medical devices, process control devices and watches. Display devices are primarily comprised of liquid crystal cells having a glass or other substrate coated with a transparent conductive material in front and behind a liquid crystal medium. Light transmission through these devices is controlled through orientation of the liquid crystal compounds or dyes dissolved therein. In this way, a voltage or, in some instances, a magnetic field may be applied to the cell so that the liquid crystals are oriented in a fashion such that all, some or none of the light is passed through. In addition, depending on the device geometry, polarizers may be used in conjunction with the liquid crystal medium to control light transmission.
Aligned liquid crystal cells in commerical use are typically oriented in directions suitable for controlling light transmission. That is, the molecules in the liquid crystal composition are aligned so as to assume a homogeneous or homeotropic alignment. Without external stimuli the display will either appear opaque or transparent. By applying an electric field the molecules are rotated along a fixed axis so as to alter the transmission properties in a desired fashion.
Current liquid crystal display elements include a product that utilizes a twisted nematic mode, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 90.degree. between a pair of upper and lower electrode substrates, a product utilizing a supertwisted nematic mode, utilizing a birefringent effect, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 180.degree. to 300.degree., and a product utilizing a ferroelectric liquid crystal substance or an antiferroelectric liquid crystal substance. Common to each of these products is a liquid crystal layer disposed between a pair of substrates coated with a polymeric alignment layer. The polymeric alignment layer controls the direction of alignment of the liquid crystal medium in the absence of an electric field. Usually the direction of alignment of the liquid crystal medium is established in a mechanical buffing process wherein the polymer layer is buffed with a cloth or other fiberous material. The liquid crystal medium contacting the buffed surface typically aligns parallel to the mechanical buffing direction. Alternatively, an alignment layer comprising anisotropically absorbing molecules can be exposed to polarized light to align a liquid crystal medium as disclosed in U.S. Pat. Nos. 5,032,009 and 4,974,941, both entitled "Process of Aligning and Realigning Liquid Crystal Media," which are hereby incorporated by reference.
The process for aligning liquid crystal media with polarized light is a noncontact method of alignment which can reduce dust and static charge buildup on alignment layers. Other advantages of the optical alignment process include high resolution control of alignment direction and high quality of alignment.
Requirements of optical alignment layers for liquid crystal displays include low energy threshold for alignment, transparency to visible light (no color), good dielectric properties and voltage holding ratios, long-term thermal and optical stability and in many applications a controlled uniform pre-tilt angle. Most liquid crystal devices, including displays, have a finite pre-tilt angle, controlled, for instance, by the mechanical buffing of selected polymeric alignment layers. The liquid crystal molecules in contact with such a layer aligns parallel to the buffing direction, but is not exactly parallel to the substrate. The liquid crystal molecules are slightly tilted from the substrate, for instance by about 2-15 degrees. For optimum performance in most display applications a finite and uniform pre-tilt angle of the liquid crystal is desirable.
The process for aligning liquid crystal media with polarized light has many attractive features. However, up to now, controlling the pre-tilt angle of liquid crystals in contact with optical alignment layers, while maintaining the uniformity of alignment, has been lacking. Furthermore to meet the above stated requirements of transparency the use of anisotropically absorbing molecules that absorb in the visible region are generally not acceptable.